Chemometric methods INDEX

One chemometric method used to monitor mixing involves comparing the spectrum for the unknown sample with that for one assumed to be homogeneous via the so-called conformity index , which is calculated by projecting the spectrum for the unknown sample onto the wavelength space of the spectrum or mean of spectra for the homogeneous sample. This procedure is similar to that involving the calculation of distances in a principal component space. 

Chemometric methods such as analysis of correlation coefficients, cluster analysis or neural network analysis are used, for example, in the classification of fragments of glass on the basis of their elemental composition or refractive index. Such methods allow the test material to be classified into the appropriate group of products on the basis of the measured parameter. 

Seven parameters of physicochemical properties, such as acid number, color, density, refractive index, moisture and volatility, saponification value and PV, were measured for quality and adnlter-ated soybean, as well as quality and rancid rapeseed oils. The chemometric methods were then applied for qualitative and quantitative discrimination and prediction of the oils by methods snch as exploratory principal component analysis (PCA), partial least squares (PLS), radial basis function-artificial neural networks (RBF-ANN), and multi-criteria decision making methods (MCDM), PROMETHEE and GAIA.260... 

Le Dreau Y, Dupuy N, Artaud J, Ollivier D, Kister J. Infrared study of aging of edible oils by oxidative spectroscopic index and MCR-ALS chemometric method. Talanta 2009 77 1748-56. 

While indexes similar to those available for gas chromatography columns have not been developed, there have been attempts to try to classify columns. One way is to use a single compound such as aspirin to characterize a series of columns for a fixed polarity analyte, to determine the relative time it elutes. Others use a chemometrics (statistical) approach to group columns according to efficiency, peak symmetry, tailing, and free silane. These methods at present are of limited value in discerning relatively small differences. The bottom line is that empirical evaluation, starting with manufacturers recommended applications, is the best way to settle on the appropriate column. 

In either case, the spectral as well as physical properties continuously change. Using refractive index or another simple device to follow mixing is never as precise as a spectral method using chemometrics. 

In recent years an entirely new class of correlative methods has been developed. These use near-infrared (NIR) or mid-infrared spectra together with sophisticated chemometric techniques to prwlict a wide variety of properties. Properties such as saturates, aromatics, and frwzing point of jet fuel [9 ] and density, viscosity, aromatics, heat of combustion, and cetane index of diesel fuel [iO] have been successfully predicted. 

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